Molibdenum (Mo) is an essential element of plants (see for example, nonpatent document 1) and its lack induces symptoms including suppression of internodal growth, or morphological abnormality of leaves (see for example, nonpatent documents 2 and 3). Mo is a transition element, which is included in an enzyme catalyzing a plurality of oxidation/reduction reaction of plants, as an electron donor or receptor. Nitrate reductase which is responsible for an important reaction in nitrogen metabolic pathway, is one of the enzymes containing Mo (see for example, nonpatent document 4).
Mo binds with these enzymes in a form of Mo cofactor (Moco) bound to a pterin compound (see for example, nonpatent document 5). Moco is essential for nitrate reductase, and the activity of nitrate reductase is low in a mutated strain in which Moco content is reduced (see for example, nonpatent document 6). Mutated strains with a low nitrate reductase activity, show a perchlorate resistant and tungstate sensitive phenotypes (see for example, nonpatent document 7). By using these phenotypes as an index, mutated strains lacking enzymes necessary for biosynthesis of Moco were isolated, and the synthesis pathway of Moco has been clarified (see for example, nonpatent document 8). On the other hand, no mutated strain in which Moco content has been lowered because of reduction of Mo concentration in plants and causal genes thereof have been reported so far.
It is thought that plants absorb Mo mainly from soil in a form of MoO42−, a bivalent negative ion (see for example, nonpatent document 9), and it is thought that a transporter intervenes in membrane penetration similarly to a general ion. In bacteria and archaea, an ABC-type (ATP-binding cassette type) Mo transporter has been identified (see for example, nonpatent document 10). However, Mo transporters have not been identified in plants.
It is known that when using Na2SO4 to grass farm containing a large amount of Mo, accumulation of Mo in pasture is suppressed (see for example, nonpatent document 11). It is thought that is because when SO42−, a bivalent negative ion similar to MoO42−, is also present in soil, Mo absorption of plants is competitively inhibited (see for example, nonpatent document 12), suggesting that MoO42− is transported from soil into plants by a mechanism similar to that of SO42−.
Sulphur is a constitutive element of amino acids, and an essential element of plants. Plants incorporate sulfur in vivo by absorbing SO42−, a bivalent negative ion, via a sulfate ion transporter (see for example, nonpatent document 13). By a sequence analysis of genomic DNA (see for example, nonpatent document 14), it was estimated that at least 14 sulfate ion transporters are present in Arabidopsis thaliana. The sulfate ion transporter family can be further classified into 5 groups according to its homology, and genes classified into groups 1 to 4 show the same characteristics in each group, for tissue-specific expression or intracellular localization (see for example, nonpatent document 15). On the other hand, the characteristics of the genes classified into group 5 have not been clarified (see for example, nonpatent document 16). A domain common in sulfate ion transporter family is present in the sequence of the genes of this group, while as a sequence as a whole, the homology with a sequence of genes belonging to other groups is low (see for example, nonpatent document 17). Further, it has not been reported that a translated product of genes belonging to group 5 has a sulfate ion transporter activity, and its function is unknown.
When comparing the element composition of Arabidopsis thaliana accessions Col-0 and Ler, Mo content in Col-0 is significantly high compared to that of Ler (see for example, nonpatent document 18).    [Patent document 1] Japanese Laid-Open patent application no. 2002-262872    [Nonpatent document 1] Amon, D. I. and Stout, P. R. (1939) The essentiality of certain elements in minute quantity for plants with special reference to copper. Plant Physiol. 14: 371/375    [Nonpatent document 2] Fido, R. J., Gundry, C. S., Hewitt, E. J. and Notton, B. A. (1977) Ultrastructural features of Molybdenum deficiency and whiptail of cauliflower leaves—effects of nitrogen-source and tungsten substitution for Molybdenum. Australian J. Plant Physiol. 4: 675-689    [Nonpatent document 3] Agarwala, S. C., Sharma, C. P., Farooq, S. and Chatterjee, C. (1978) Effect of Molybdenum deficiency on the growth and metabolism of corn plants raised in sand culture. Can. J. Bot. 56: 1905-1908    [Nonpatent document 4] Mendel, R. R. and Hansch, R. (2002) Molybdoenzymes and Molybdenum cofactor in plants. J. Exp. Bot. 53: 1689-1698    [Nonpatent document 5] Johnson, J. L., Hainline, B. E. and Rajagopalan, K. V. (1980) Characterization of the Molybdenum cofactor of sulfite oxidase, xanthine, oxidase, and nitrate reductase. Identification of a pteridine as a structural component. J. Biol. Chem. 255: 1783-1786    [Nonpatent document 6] Gabard, J., Pelsy, F., Marionpoll, A., Caboche, M., Saalbach, I., Grafe, R. and Muller, A. J. (1988) Genetic-analysis of nitrate reductase deficient mutants of Nicotiana-plumbaginifolia—evidence for 6 complementation groups among 70 classified Molybdenum cofactor deficient mutants. Mol. Gen. Genet. 21/3: 206-21/3    [Nonpatent document 7] LaBrie, S. T., Wilkinson, J. Q., Tsay, Y. F., Feldmann, K. A. and Crawford, N. M. (1992) Identification of two tungstate-sensitive Molybdenum cofactor mutants, chl2 and chl7, of Arabidopsis thaliana. Mol. Gen. Genet. 233:169-176    [Nonpatent document 8] Mendel, R. R. (1997) Molybdenum cofactor of higher plants: biosynthesis and Molecular biology. Planta 203: 399-405    [Nonpatent document 9] Gupta, U. C. and Lipsett, J. (1981) Molybdenum in soils, plants, and animals. Adv. Agron. 34: 73-115    [Nonpatent document 10] Self, W. T., Grunden, A. M., Hasona, A. and Shanmugam, K. T. (2001) Molybdate transport. Res. Microbiol. 152: 311/321    [Non patent document 11] Chatterjee, C., Nautiyal, N. and Agarwala, S. C. (1992) Excess sulphur partially alleviates copper deficiency effects in mustard. Soil Sci. Plant Nutr. 38: 57-64    [Nonpatent document 12] Pasricha, N. S., Nayyar, V. K., Randhawa, N. S. and Sinha, M. K. (1977) Influence of sulphur fertilization on suppression of Molybdenum uptake by berseem (Trifolium alexandrinum L.) and oats (Avena sativa L.) grown in a Molybdenum-toxic soil. Plant Soil 46: 245-250    [Nonpatent document 13] Smith, F. W., Ealing, P. M., Hawkesford, M. J. and Clarkson, D. T. (1995) Plant members of a family of sulfate transporters reveal functional subtypes. Proc. Natl. Acad. Sci. USA 92: 9373-9377    [Nonpatent document 14] The Arabidopsis Genome Initiative (2000) Analysis of the genome sequence of the flowering plant Arabidopsis thaliana. Nature 408: 796-815    [Nonpatent document 15] Hawkesford, M. J. (2000) Plant responses to sulfur deficiency and the genetic manipulation of sulfate transporters to improve S-utilization efficiency. J. Exp. Bot. 51: 1/31-1/38    [Nonpatent document 16] Hawkesford, M. J. (2003) Transporter gene families in plants: the sulphate transporter gene family. Redundancy or specialization Physiologia Plantarum 117: 155-163    [Non patent document 17] Buchner, P., Takahashi, H. and Hawkesford, M. J. (2004) Plant sulphate transporters: co-ordination of uptake, intracellular and long-distance transport. J. Exp. Bot. 55: 1765-1773    [Nonpatent document 18] Lahner, B., Gong, J., MahMoudian, M., Smith, E. L., Abid, K. B., Rogers, E. E., Guerinot, M. L., Harper, J. F., Ward, J. M., McIntyre, L., Schroeder, J. I. and Salt, D. E. (2003) Genomic scale profiling of nutrient and trace elements in Arabidopsis thaliana. Nat. Biotechnol. 21: 1215-1221